This invention relates in general to the field of supercapacitors. More particularly, this invention relates to a superior supercapacitor having electrodes fabricated from specially synthesized nanostructured carbon (SNC) powders in which the pore sizes and the specific surface may be selectively controlled.
Electric double layer capacitors are widely used in a variety of industries. For example, PCT WO99/24995 shows an electric double layer capacitor and manufacturing method. The electric double layer capacitor comprises metal foil or mesh current collectors, polarizable electrodes made of an activated carbon and a polymer binder, conductive coatings comprising a melamine resin binder at the interfaces between current collectors and polarizable electrodes, and non-aqueous electrolyte. The thickness of the polarizable electrodes is 15 micron. The electrodes were deposited into the conductive layer coated current collector by a screen printing method. Such a electric double layer capacitor demonstrates long term low impedance at the electrode-current collector interface and high power capabilities.
Another prior art capacitor is described in U.S. Pat. No. 5,150,283, showing an electric double layer capacitor and method for producing the same. The method includes depositing a mixture prepared by dispersing activated carbon powder and agent for improving the electric conductivity of the layer on an aluminum substrate by either means of spreading, impregnating or printing. The layer thickness is equal to 50-100 micron.
U.S. Pat. No. 5,776,633 describes carbon/carbon composite materials and use thereof in electrochemical cells. This invention provides an activated carbon fabric impregnated with a mixture of activated carbon powder and binder; the thickness of materials obtained being 125-250 micron. The advantage of materials obtained includes the low resistivity, the ability to easily form thin composites with very good mechanical strength.
Another example of an electric double layer capacitor is described in U.S. Pat. No. 5,142,451. Specifically, an electric double layer capacitor which comprises a plurality of thin plate-like electrode elements is disclosed. The polarizable electrodes were manufactured by sintering powder of minute active carbon particles having an average diameter of about 20 micron into a porous sintered electrode body. This polarizable electrode is held in contact with a current collector through an electrically conductive layer to reduce the inner resistance of the capacitor.
A method for manufacturing a polarizable electrode for electric double layer capacitor is taught in U.S. Pat. No. 5,277,729. The thickness of electrode is about 40-500 micron. The electrode is obtained by hot rolling an initial mixture of fine carbon powder, polymer resin and liquid lubricant.
A metal electrode material, capacitor using metal electrode material, and method of manufacture is disclosed in PCT WO 99/38177. The metal electrode material comprises a xe2x80x9cvalve metal materialxe2x80x9d with carbon particles on its surface. The metal electrode material is coated with an activated carbon layer and used as polarizable electrode for an electric double layer capacitor. That capacitor has reduced inner resistance, resulting in an increase in capacitance.
U.S. Pat. No. 5,742,474 describes an electric double layer capacitor, comprising a pair of polarized electrodes made of the same activated carbon materials. However, the amount of the carbon material of the polarized positive electrode is higher than that of the polarized negative electrode. The main advantage is that it is possible to increase a voltage applied to the electric double layer capacitor.
The vast majority of carbon materials used for electrodes in electric double layer capacitors (EDLC) have been prepared by the charring or carbonization of organic substances, usually followed by a surface activation process using water vapor or other activation agent.
The foregoing demonstrates that electrodes and capacitors have been widely studied in the prior art. Yet with all of this study, there is still a great need for the development of supercapacitors that exhibit superior performance.
For the purpose of this patent application, the terms nanoporous, nanoporosity and nanostructured apply to pore sizes less than 3 nanometer. By transport porosity is meant pores larger than 3 nanometers.
In summary, an object of the present invention is to provide a supercapacitor, which exhibits superior performance. In particular, the present invention provides the following features. Specially synthesized nanostructured carbon (SNC) powder is processed to fabricate electrodes in such a manner that the resultant electrodes have pore sizes, which are selectively and closely controlled. Further, thin, composite aluminum and SNC electrodes are made using the carbon powder. Additionally, by the ability to selectively control the resultant pore sizes in the electrodes, a capacitor is provided wherein the positive and negative electrodes are balanced with respect to their nanopore size and active carbon content, thereby tailoring the electrodes to fit the ionic sizes of the electrolyte positive and negative ions respectively employed with the capacitor. This also allows, in another aspect of the present invention, the selection of the most efficient electrolyte with respect to its conductivity and other desirable features. The present invention also provides for the selection of a desirable separator, which gives increased conductivity and leaves sufficient free ion concentration when charged.
In another aspect of the present invention, a method is provided wherein SNC is synthesized from inorganic polycrystalline material to selectively control the pore size and pore size distribution in the resulting electrode.
In one embodiment, an electric double layer capacitor is provided comprising: at least two thin and flexible polarizable electrodes obtained by rolling a mixture of SNC material with a binder. Said electrodes are connected to metal current collectors; a thin layer of a porous, ion-permeable but electron-insulating material (separator) interposed between electrodes; and a liquid electrolyte.
In another embodiment, an electric double layer capacitor is provided comprising: a pair of polarizable electrodes made of a SNC material having different porosities (and pore size distributions); a thin layer of a porous, ion-permeable but electron-insulating material (separator) interposed between the electrodes; and a liquid electrolyte.